Catalyst and catalyst carrier

ABSTRACT

The carrier of the present invention includes at least 85 wt percent alpha alumina, at least 0.06 wt percent SiO 2  and no more than 0.04 wt percent Na 2 O. The carrier has a water absorption no greater than 0.35 g/g and a ratio of water absorption (g/g) to surface area (m 2 /g) no greater than 0.50 g/m 2 . Another aspect of the invention is a catalyst for the epoxidation of olefins which comprises the above described carrier and silver dispersed thereon, where the carrier has a monomodal, bimodal or multimodal pore distribution and where the quantity of silver is between 5 and 50 wt %, relative to the weight of the catalyst. A reactor system for the epoxidation of olefins is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/547,089 filed Oct. 14, 2011.

FIELD OF THE INVENTION

This invention generally relates to a carrier and a catalyst made fromthe carrier. More particularly, this invention is concerned with analumina based carrier and a catalyst useful in the production of anolefin oxide, a 1,2-diol, a 1,2-diol ether, 1,2-carbonate, or analkanolamine.

BACKGROUND OF THE INVENTION

In olefin epoxidation, feedstocks containing an olefin and an oxygensource are contacted with a catalyst disposed within a reactor underepoxidation conditions which results in the production of olefin oxideand typically includes unreacted feedstock and combustion products. Thecatalyst usually comprises a catalytically active material, such assilver, deposited on a plurality of ceramic pellets which may bereferred to as carrier. Processes for making carrier are described inU.S. Pat. No. 6,831,037 and U.S. Pat. No. 7,825,062.

The technology used to manufacture carriers that are desirable for useas catalyst supports in an olefin epoxidation reaction has evolvedsubstantially over the last few decades. In U.S. Pat. No. 4,007,135(Hayden), which issued on Feb. 8, 1977, the description of example 4discloses a carrier sold by Norton Co. wherein the “porosity to waterwas 25%” and the surface area of the carrier was 0.36 m²/g. Thedescription of example 7 in Hayden discloses a support which had a waterporosity of 16 to 20% and a surface area of 0.17 m²/g. In contrast tothe descriptions in examples 4 and 7 in the Hayden reference, which maybe generally described as disclosing carriers having low surface areaand low pore volume, U.S. Pat. No. 5,187,140 (Thorsteinson), whichissued on Feb. 16, 1993, discloses “a high surface area, high porositycarrier” (see column 6, lines 32-33) for the epoxidation of alkene toalkylene oxide. In column 7, lines 40-51, Thorsteinson describes thecarriers of the subject invention as having a surface area greater thanabout 0.7 m²/g and, preferably, having a water pore volume of at leastabout 0.55 cc/g and most preferably from about 0.6 to about 0.8 cc/g.The '140 reference also discusses the teachings of the EP 0,327,356(Jin); and U.S. Pat. No. 4,829,043 (Boehning) in the Background of theInvention section of the specification. The Jin reference is describedas disclosing a carrier having “a total pore volume greater than 0.5milliliters per gram, preferable 0.5 to 0.7 milliliters per gram” and “asurface area of 0.2 to 2 m²/g, preferably 0.8 to 1.3 m²/g”. The Boehningreference is described as disclosing a carrier that “has a surface areaof 0.4 to 0.8 m²/g and a pore volume of not less than 0.45 milliliterper gram.” While the information in these references generally indicatesthat the technology used to manufacture carriers for catalysts used inthe production of alkylene oxides has evolved from dense (i.e. low porevolume) and low surface area carriers to porous (i.e. high pore volume)and high surface area carriers there have been a few disclosures of lowpore volume, high surface area carriers. For example, the '140 referenceidentified above also discloses CARRIER “AC” which was described as“available from the Norton Company, Stow, Ohio as 5502” and had asurface area of 0.80 m²/g and water pore volume of 0.26-0.32 cc/g. Inanother reference, Example 1A in US 2009/0192324 discloses an alphaalumina carrier having the following characteristics “(specific surfacearea: 1.0 m²/g; water absorption: 35.7% by weight; SiO₂ content: 3.0% byweight; Na₂O content: 0.35% by weight;”. The general trend in thetechnical evolution of carriers described above, which has continued forapproximately two decades, is believed to have occurred because thedisclosed carriers did not provide the desired performance when used asa catalyst support.

A key driver behind the technical efforts to provide an improvedcatalyst has been to reduce the manufacturing cost of a reactor's finalproduct (i.e. an olefin oxide) such as ethylene oxide. The cost ofmanufacturing can be impacted, both positively and negatively, inseveral ways which may be interrelated and thus complicated to isolateand improve upon. For example, the cost of the final product can bereduced if the selectivity of the reaction can be increased without acorresponding increase in the reactor's operating temperature. As usedherein, selectivity is an indication of the proportion, usuallyrepresented by a percentage, of the converted material or product whichis alkene oxide. If the carrier and catalyst can be changed so that theselectivity of the reactor is improved, then a higher percentage of thereactants are converted to the desired final product relative to thepercentage of reactants converted with a previously used catalyst. Thecost of the final product may also be reduced if the reactor's operatingtemperature can be reduced relative to another carrier that hasgenerally equivalent or lower selectivity. Another tactic to reduce thecost of the final product is to improve the longevity of the catalystwhich means that the reactor can be operated for longer periods of timebefore the selectivity and/or activity of the catalyst declines and/orthe temperature increases to an unacceptable level which requires thereactor to be stopped so that the catalyst can be replaced. Stopping thereactor to replace the catalyst inherently incurs expenses that add tothe cost of the final product.

With regard to the evolution of carrier and catalyst technology, theinventors of this application have recognized that there is a strongsymbiotic relationship between changes made to the carrier andsubsequent changes made to the catalyst which collectively improve ordegrade the economic performance of the reactor. For example, asdescribed above, some commercially available carriers have had low porevolumes, such as less than 0.35 g/g of catalyst, which may have limitedthe amount of catalytically active material (i.e. silver) which could bedeposited. Limiting the amount of silver per gram of catalyst inherentlylimited the amount of silver per unit of volume within the reactor.However, carriers with total pore volume below 0.35 g/g, which may alsobe described as high density carriers, were resistant to crushing andabrasion which were desirable characteristics. Furthermore, the chemicalcomposition of the carrier was substantially influenced by theimpurities in the commercially available raw materials used to make thecarrier. Some of the raw materials were the alumina, bond material andpore formers. Each of the raw materials had the potential tointentionally (or unintentionally) import excessive levels of certaincompounds, such as Na₂O, SiO₂ and potassium containing compounds whichcould adversely impact the performance of the catalyst. To improve theperformance of the catalyst researchers began to develop carriers thatwere more porous than their predecessors thereby increasing the amountof silver which could be deposited. Evidence of the move to developingmore porous carriers can be found in the teachings in U.S. Pat. No.7,547,795 (Matusz) which describes carriers of similar surface area, butwith varying water absorption values. Furthermore, this patent teachesthat increasing the water absorption of the carrier “allows for theloading of a greater amount of silver onto the support material than canbe loaded onto other inorganic materials that have a lower waterabsorption.” As the amount of silver per gram of carrier is increased,the amount of silver per unit of volume within the reactor is alsoincreased which leads to improved selectivity and longevity.Unfortunately, increasing the porosity of the carrier reduces thecarrier's resistance to crushing and increases its abrasion which areboth undesirable attributes.

The goal of producing a carrier and catalyst that is both resistant tocrushing and abrasion and enables selectivities and longevities beyondthose commercially available has heretofore been difficult to achievebecause of the perceived conflict between making a carrier with goodresistance to crushing and abrasion while also providing useful porosityto allow for enough silver to be deposited onto a carrier andsubsequently loaded into a reactor. The inventors of the inventiondescribed and claimed below have recognized that a carrier havingcertain micro physical and chemical characteristics, as will beexplained, can improve the selectivity of the catalyst while providing aphysically robust carrier thereby reducing the cost of the desired finalproduct.

SUMMARY

Embodiments of the present invention provide a physically robust carrierthat can withstand the crushing and abrasion forces typicallyexperienced by a carrier during the carrier and catalyst manufacturingprocesses while also providing usable porosity and surface area andwithout the need to incorporate therein raw materials that includeimpurities which negatively impact the performance of a catalyst madefrom the carrier.

In one embodiment, the carrier of the present invention includes atleast 85 wt percent alpha alumina, at least 0.06 wt percent SiO₂ and nomore than 0.04 wt percent Na₂O. The carrier has a water absorption nogreater than 0.35 gram of water/gram of carrier and a ratio of waterabsorption (gram of water/gram of carrier) to surface area (m² ofcarrier/gram of carrier) no greater than 0.50 gram of water/m² ofcarrier.

In another embodiment, this invention is a catalyst for the epoxidationof olefins. The catalyst comprises the above described carrier andsilver dispersed thereon, where the carrier has a monomodal, bimodal ormultimodal pore distribution and where the quantity of silver is between5 and 50 wt %, relative to the weight of the catalyst.

In still another embodiment, this invention is a reactor system for theepoxidation of ethylene, which reactor system comprises at least oneelongated tube having an internal tube diameter of between 20 and 50 mm,wherein contained is a catalyst bed of catalyst particles comprisingsilver in a quantity between 5 and 50 wt %, relative to the weight ofthe catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A is a graph of total pore volume curves;

FIG. 1 B is a graph of incremental pore volume curves;

FIG. 2 A is a graph of total pore volume curves; and

FIG. 2 B is a graph of total incremental volume curves.

DETAILED DESCRIPTION

Porous ceramic bodies used as carriers for catalytically active materialhave numerous physical and chemical characteristics that collectivelyand individually influence the selectivity, longevity, yield anddurability of the catalyst when disposed in a chemical reactor. Theporous body's physical and chemical characteristics may also impact themanufacturability of the carrier and the catalyst. Numerous patents andtechnical articles have focused on improving the catalyst by modifyingcharacteristics such as the carrier's surface area, water absorption,pore size distribution and morphology, which may be referred to hereinas the carrier's micro physical characteristics. In other publications,the carrier's macro physical characteristics, such as its crushstrength, resistance to abrasion, length, outer diameter and innerdiameter, have been described. In yet other publications, the carrier'schemical characteristics, such as the amounts of potassium and silica,have been described. This invention describes a carrier and catalystmade therefrom which have a unique blend of micro physicalcharacteristics and chemical characteristics that result in a catalystwhich is both physically robust and provides the desired selectivitywhen used in a chemical reactor.

A carrier suitable for use as a substrate for catalytically activematerial has a functional lifetime, as defined herein, which begins whenthe carrier is formed as a discrete non-sintered pellet, known asgreenware, and ends when the catalyst, which is formed from the carrier,is removed from a reactor. Many ceramic carriers, including carriersused to make catalyst for epoxidation processes, are exposed to variousmanufacturing processes and environmental conditions during theirlifetime which could negatively impact the performance of the catalystin a chemical reactor. The processes and environmental conditionsdescribed below could impact the performance of the catalyst by alteringthe physical and/or chemical characteristics of the catalyst in anundesirable manner.

From a physical point of view, two of the carrier's fundamental macrophysical characteristics are its crush resistance and resistance toabrasion. To be commercially viable carriers should be sufficientlyrobust to withstand crushing and abrasion which may occur during one ofseveral processing steps. For example, during the carrier manufacturingprocess, the carrier may be formed via an extrusion process thatproduces the greenware which may be small tubularly shaped pellets thatare easily deformed by squeezing the greenware between a person'sfingers. In commercial processes, the greenware may be loaded into largekiln cars which hold thousands of greenware pellets piled randomly ontop of one another. The greenware at the bottom of the car must be ableto withstand crushing by the greenware located directly above it in theupper regions of the car. The cars may be made to pass through a largekiln where the pellets are sintered thereby producing ceramic carriersthat are both rigid and potentially frangible if sufficient force isexerted on the carrier. The carriers may then be physically removed fromthe cars and stored in large containers, such as steel drums, forstorage and subsequent shipment by truck which may subject the carriersto frequent impacts during transit. A carrier's resistance to abrasionmay be measured using ASTM D4058-96.

A carrier's surface area and water absorption are two micro physicalcharacteristics commonly used to characterize a carrier. The carrier'ssurface area is an indication of the amount of surface area per gram ofcarrier available for the deposition of the catalytically activematerial. The surface area may be determined using the proceduredescribed by BET (Brunauer, Emmett and Teller) method as described inJournal of the American Chemical Society 60 (1938) pp. 309-316. Thecarrier's water absorption may be an indication of the carrier's abilityto absorb fluids such as the fluids used in the catalyst preparationprocess to deposit catalytically active metal, promoters andco-promoters onto the carrier's available surface area. Water absorptionmay be measured using the following procedure. First, approximately 100g of representative samples of carrier are dried at 110° C. for onehour. The samples are then cooled in a desiccator and the dry mass (D)of each sample is then determined to the nearest 0.01 g. The samples arethen placed in a pan of distilled water and boiled for thirty minutes.While the water is boiling, the samples are covered with water andsetter pins or some similar device are used to separate the samples fromthe bottom and sides of the pan and from each other. After the thirtyminute boil the samples are allowed to soak for an additional fifteenminutes. After returning to room temperature each sample is then blottedlightly with a moistened, lint-free linen or cotton cloth to remove allexcess water from the surface and the saturated mass (M) is determinedto the nearest 0.01 g. The blotting operation may be accomplished byrolling the specimen lightly on the wet cloth which shall previouslyhave been saturated with water and then pressed only enough to removesuch water as will drip from the cloth. Excessive blotting should beavoided because it will introduce error by withdrawing water from thepores of the sample. The samples should be weighed immediately afterblotting. The entire operation should be completed as quickly aspossible to minimize errors caused by evaporation of water from thesample. Water absorption (A) is the relationship of the mass of waterabsorbed to the mass of the dried carrier and is determined using thefollowing formula: A=[(M−D)/D]×100 wherein the water absorption isexpressed as a percent of the weight of the carrier. Water absorptionmay also be expressed as the weight of the water that can be absorbedinto the pores of the carrier relative to the weight of the carrier andtherefore reported as grams of water per gram of carrier and the unitsmay be abbreviated as “g/g”. Water absorption may also be expressed ascc/g provided there is a correction for the density of water at theconditions measured. A carrier's water absorption value may bepositively correlated to and thus used interchangeably with the term“porosity” which, in the field of catalyst carriers, is usuallyunderstood to mean the carrier's open cell porosity. As a general rule,there is an inverse correlation between water absorption and crushstrength.

Recent trends in ethylene oxide catalyst manufacturing have utilizedcarriers with increasingly higher surface areas and water absorptions.The latter is typically achieved by incorporation of variouspore-forming materials into the carrier mix before firing into formedceramic bodies, which can impart undesired properties to the finishedcarrier. One effect of increasing the pore forming agent is theweakening of the formed pellet, which can manifest itself in lower flatplate crush strength or decreased resistance to attrition duringhandling. Particularly in the case of fixed surface area with increasingwater absorption, the resulting catalyst after impregnation with metalcontaining solution and drying will feature increasingly higher silversurface density. This is a direct result of depositing higher amounts ofmetal onto a fixed surface area. Without wishing to be bound by theory,it is thought that the increased crowding of metal onto carrier supportsurfaces promotes the process of sintering of the metal particles, andthus results in loss of catalyst activity. In the current invention,this effect is thought to be mitigated by minimizing the larger poresformed by pore forming agents and accommodating the need for targetmetal loadings through multiple impregnations of metal containingsolution. In this manner, with multiple impregnations on a low waterabsorption carrier, one can attain metal loadings which are equivalent,based on the mass of metal per unit volume of bulk catalyst, to thoseobtained with fewer impregnations on a higher water absorption carrier

Another micro physical characteristic is the carrier's pore sizedistribution. The pore size distribution may be measured by aconventional mercury intrusion porosimetry device in which liquidmercury is forced into the pores of a carrier. Greater pressure isneeded to force the mercury into the smaller pores and the measurementof pressure increments corresponds to volume increments in the porespenetrated and hence to the size of the pores in the incremental volume.As used herein, the pore size distribution, the median pore diametersand the pore volumes are as measured by mercury intrusion porosimetry toa pressure of 2.1×10⁸ Pa using a Micromeretics Autopore 9200 model (130°contact angle, mercury with a surface tension of 0.480 N/m, andcorrection for mercury compression applied). As used herein, the medianpore diameter is the pore diameter at which half of the total porevolume is contained in pores having a larger pore diameter and half ofthe total pore volume is contained in pores having a smaller porediameter.

After the carriers have been manufactured and transported to a catalystmanufacturing facility, they may be exposed to an additional set ofphysical impacts during the catalyst manufacturing process. For example,after the carriers have been removed from the shipping containers andbegin processing through the catalyst manufacturing process they may beexposed to high centrifugal force during a chemical impregnation processwhich causes individual carriers to collide with other carriers and theinterior surface of metal equipment. The force exerted on the catalystmay cause the catalyst to break and/or abrade thereby producing a fineceramic powder that reduces the quantity of usable catalyst and may clogthe catalyst manufacturing equipment. After the catalyst has beenmanufactured, the catalyst pellets may be dropped into tube shapedreactors that may be between 3 and 25 meters in length and 20 mm to 50mm in diameter. The thickness of the tube walls may be between 0.5 and10 mm. If the catalyst pellets break during loading into the reactor thepieces of catalysts could negatively impact the performance of thereactor by increasing the pressure drop, altering the flow of thereactants and byproducts through the reactor, and exposing catalystsurface that does not contain the catalytically active material. Abradedand broken catalyst pellets may cause a reduction in the efficiency ofthe reactor thereby increasing the cost of the final product.

From a chemical point of view, a carrier's chemical composition may beinfluenced by several factors including impurities in the raw materialsused to make the carriers. An example of a common raw material isalumina, such as alpha alumina, in powder form which is a well-knowningredient for manufacturing catalysts for the production of ethyleneoxide and other epoxidation reactions. The impurities in the alphaalumina may depend on the process used to manufacture the alpha alumina.Another class of raw materials known as bond materials typically containa mixture of elements and compounds that serve to bind the particles ofalumina powder into discreet, self-supporting greenware or as a sinteredcarrier. The phrase “bond material” may include temporary bond materialand/or permanent bond material. Temporary bond material, such aspolyolefin oxides, celluloses and substituted celluloses, includingmethylcellulose, starch, ethylcellulose and carboxyethylcellulose,typically enable the greenware to remain intact during the carriermanufacturing process. In contrast to temporary bond materials,permanent bond material usually remains a part of the carrier after ithas been sintered. Examples of permanent bond materials include alkalineearth metal compounds and alkali metal compounds. Preferably, thealkaline earth metal compounds include silicates such as magnesiumsilicate, calcium silicate and barium silicate. Unfortunately both thetemporary bond materials and the permanent bond materials may containone or more impurities that negatively impact the performance of thecatalyst. Another class of raw materials is commonly known as poreformers which are used to induce a desired amount of porosity having acertain pore size distribution. The pore formers are typically removedfrom the carrier during the sintering of the carrier. The pore formersmay be naturally occurring material or manufactured materials. Anexample of a naturally occurring material is comminuted shells of nutssuch as pecan, cashew, walnut, peach, apricot and filbert which may bereferred to herein as coarse pore formers. Examples of syntheticmaterials are polypropylene and/or polyethylene. The quantities andvarieties of chemical impurities in the naturally occurring materialsare inherently more variable than the quantities and varieties ofchemical impurities in the manufactured bond material. Consequently, theresidue that remains in a carrier after the naturally occurring porematerial has been burned out during sintering may contain a variablenumber of impurities that can adversely impact the selectivity andlongevity of the catalyst. Impurities commonly introduced into thecarrier by the pore former include potassium containing compounds.Depending on the combinations and concentrations of the impurities, theimpurities may only slightly or, in contrast, significantly impact theperformance of the catalyst made therefrom. Other raw materials used tomanufacture carriers are fluids such as solvents and extrusion aids.Water, particularly de-ionized water, is the most common solvent. Theamount of water used in a particular mix is adjusted to achieve adesired flowability through an extrusion die. Typical quantities ofwater vary from 10 weight percent to 60 weight percent based on theweight of the alumina. Examples of suitable extrusion aids includepetroleum jelly, grease, polyolefin oxides and polyethylene glycol.

Carriers for olefin epoxidation catalysts can be made by differentprocesses that result in carriers having distinct morphologies. In afirst process, which is disclosed in U.S. Pat. No. 4,994,589, carrier ismade by a process that produces alpha-alumina support particles having a“platelet morphology”. FIG. 1 in U.S. Pat. No. 4,994,589 is a scanningelectron micrograph of alpha-alumina support particles having a plateletmorphology. To produce carrier with the platelet morphology, a “fluorinerecrystallizing agent is used in an amount sufficient to effectconversion of the alumina to alpha-alumina having at least onesubstantially flat surface.” “The “substantially flat major surface”referred to herein may be characterized by a radius of curvature of atleast about twice the length of the major dimension of the surface.Preferably, the particles also have aspect ratios of at least about 4:1,the aspect ratio being the ratio of the longest or major dimension tothe smallest or minor dimension.” The process forms alumina having theplatelet morphology which, when viewed at high magnification such as2000×, approximates the shapes of “small plates or wafers”. As describedin U.S. Pat. No. 4,994,589, “A portion of the support particlespreferably are formed as “interfused” or “interpenetrated” platelets,that is, having the appearance of platelets growing out of or passingthrough one another at various angles.” With regard to the quantity ofplatelet alumina in the carrier, “Preferably, at least about 50 percentof particles of the support having a particle size of at least 0.1micron comprise particles having at least one substantially flat majorsurface.” Furthermore, “These platelet-type particles frequently havesubstantially angular edge portions, as contrasted with amorphous orrounded edge portions of conventional support materials, includingconventional alpha-alumina supports.” In a second process,“conventional” carrier, which may be referred to herein as carriercomprising non-platelet alumina, is made without using a fluorinerecrystallizing agent. As described herein, carrier comprisingnon-platelet alumina, which is also known as non-platelet carrier, hasvery few, if any, particles of alumina having at least one substantiallyflat major surface. As used herein, no more than 25 percent of thenon-platelet carrier's alumina particles have at least one substantiallyflat major surface. The second process typically uses small amounts ofone or more bond materials to facilitate bonding of the aluminaparticles to one another. The bond material may partially coat some ofthe alumina particles and/or may appear to accumulate between theparticles thereby forming bond posts. The morphology of the carrier madeby the second process impacts physical characteristics of the carrier,such as surface area, water absorption, pore size distribution andparticle size.

Inventors of the invention claimed herein have developed andcharacterized carriers that enable the production of high selectivitycatalyst which are also sufficiently robust to withstand the rigors towhich a commercially available carrier is exposed during its functionallifetime. Carriers of the present invention are devised to incorporatetherein a minimum amount of silica, measured as SiO₂, and less than amaximum amount of Na₂O. The carriers also have less than a maximumamount of water absorption and the carrier's ratio of water absorptionto surface area does not exceed a specified maximum. Carriers having theunique combination of chemical and physical attributes and a processthat can be used to make the carriers will now be described.

In one embodiment, a carrier of the present invention has at least 85weight percent alumina, at least 0.06 weight percent silica measured asSiO₂, and no more than 0.04 weight percent Na₂O. The percentage ofalumina, based on the total weight of the carrier, may be 90 weightpercent, 95 weight percent or higher. The quantities of SiO₂ and Na₂Oare determined using Inductively Coupled Plasma-Optical EmissionSpectroscopy (ICP-OES) analysis, wherein the samples are prepared usinga fusion process, and are based on the total weight of the carrier afterthe carrier has been sintered and before the start of any subsequentprocessing steps that could alter the chemical composition of thecarrier. As used herein, the phrase “subsequent processing steps”includes, for example, processes such as wash coating, rinsing,immersion in a liquid, or deposition of an element or compound on thesurface of the carrier. The amount of silica in the carrier could bebetween 0.06 to 0.40 weight percent, such as, 0.08, 0.15, 0.18, 0.20,0.30 or 0.35 weight percent. Similarly, the amount of Na₂O could bebetween 0.01 and 0.04 weight percent, such as 0.02 or 0.03 weightpercent Unlike some carriers in the prior art that may meet one of thelimitations described above, the combination of a minimum amount ofsilica and no more than a maximum amount of Na₂O is believed tocontribute to the creation of a high selectivity catalyst.

With regard to physical characteristics, in one embodiment a carrier ofthis invention may have a water absorption value no greater than 0.35gram of water/gram of carrier which may be abbreviated as 0.35 g/g, anda ratio of water absorption to surface area no greater than 0.50 gram ofwater/m² of carrier which may be abbreviated as 0.50 g/m². In someembodiments, a carrier of this invention may have a water absorptionless than 0.35 g/g, such as 0.32 or even 0.30 g/g and the ratio of waterabsorption to surface area may be no greater than 0.45 or 0.40 g/m². Theratio of water absorption to surface area is determined by measuring thecarrier's water absorption as grams of water per gram of carrier andthen dividing the water absorption by the carrier's surface area whichis measured as m²/g. The combined use of: (1) water absorption; and (2)the ratio of water absorption to surface area inherently limits thesurface area of a carrier that has a 0.35 g/g water absorption value tono less than 0.70 m²/g. In some embodiments, the carrier's surface areacould be 0.75, 0.80, 0.85 m²/g and higher. Intermediate surface areassuch as 0.78, 0.82 and 0.90 m²/g are feasible and contemplated. Thecombined use of water absorption and the ratio of water absorption tosurface area also provides for carriers that have a water absorptionless than 0.35 g/g to have a surface area less than 0.70 m²/g. Forexample, if a carrier has a water absorption value of 0.25 g/g then thesurface area could be 0.50 m²/g and the carrier would have a ratio ofwater absorption to surface area of 0.50 g/m². In contrast to the lowpore volume and low surface area carriers disclosed by Hayden and thehigh pore volume and high surface area carriers disclosed byThorsteinson, carriers of this invention may be generally described aslow pore volume and high surface area carriers.

In some embodiments, the pore size distribution of a carrier of thisinvention may have a majority of the carrier's total pore volumecontributed by pores having diameters within a narrow range. Forexample, at least 60 percent of the total pore volume could becontributed by pores within a range of 3.8 microns. In some embodiments,at least 80 percent, 90 percent or more of the total pore volume couldbe contributed by pores within a range of 3.8 microns. Furthermore, nomore than 10, 15 or even 20 percent of the total pore volume may becontributed by pores having a diameter greater than 10 microns.Controlling the pore size distribution of carriers of this invention todistributions wherein the majority of the total pore volume iscontributed by pores within a narrow range and limiting the amount ofpore volume contributed by large pores (i.e. greater than 10 microns)may help to achieve the desired low pore volume and high surface areacharacteristics.

The catalyst pellets may have a number of different shapes, with themost common shape being a small cylinder pellet shape with a hole in thecenter of the pellet. Other possible shapes are disclosed in WO2004/014549; U.S. Pat. No. 2,408,164 and EP 1,184,077 A1. Preferably,the catalyst particles have a generally hollow cylinder geometricconfiguration having a length of from 4 to 20 mm, an outside diameter offrom 4 to 20 mm, an inside diameter of from 0.1 to 6 mm and a ratio ofthe length to the outside diameter in the range of from 0.5 to 2.

Preparation of Silver Catalysts

The preparation of the silver catalyst is known in the art and the knownmethods are applicable to the preparation of the catalyst which may beused in the practice of this invention. Methods of depositing silver onthe carrier include impregnating the carrier or carrier bodies with asilver compound containing cationic silver and/or complexed silver andperforming a reduction to form metallic silver particles. For furtherdescription of such methods, reference may be made to U.S. Pat. No.5,380,697; U.S. Pat. No. 5,739,075; U.S. Pat. No. 4,766,105 and U.S.Pat. No. 6,368,998, which are incorporated herein by reference.Suitably, silver dispersions, for example silver sols, may be used todeposit silver on the carrier.

The reduction of cationic silver to metallic silver may be accomplishedduring a step in which the catalyst is dried, so that the reduction assuch does not require a separate process step. This may be the case ifthe silver containing impregnation solution comprises a reducing agent,for example, an oxalate, a lactate or formaldehyde.

When catalysts of different silver contents are prepared on supportmaterials of similar packing densities it is convenient to compare themon a silver weight basis, which is typically expressed in weight percentsilver as a function of the total weight of catalyst.

Appreciable catalytic activity is obtained by employing a silver contentof the catalyst of at least 1 wt %, relative to the weight of thecatalyst. Preferably, the catalyst comprises silver in a quantity offrom 5.0 to 50.0 wt %, more preferably from 7.5 to 45.0 wt %, forexample 10.5 wt %, or 12.0 wt %, or 19.0 wt %, or 25.0 wt %, or 35.0 wt%. As used herein, unless otherwise specified, the weight of thecatalyst is deemed to be the total weight of the catalyst including theweight of the carrier and catalytic components, for example silver,rhenium promoter, first and second co-promoters and further elements, ifany.

Alternatively, the silver loading can be expressed in terms of mass ofsilver per unit volume of catalyst as it is loaded into the reactortubes. In this way, comparisons of silver loadings between catalystsprepared on support materials of very different bulk packing densitiescan be made. Ultimately catalyst is loaded into reactor tubes in adefined volume, so this method of comparing silver loadings is mostappropriate. Preferably, silver content expressed in this manner are atleast 50 kg/m³, relative to the volume of a packed bed of the catalyst.Preferably, the catalyst comprises silver in a quantity of from 50 to500 kg/m³, more preferably from 100 to 450 kg/m³, for example 140 kg/m³,or 220 kg/m³, or 230 kg/m³, or 250 kg/m³, or 300 kg/m³. As used herein,unless otherwise specified, the weight of silver is deemed to be theweight of silver contained in one cubic meter of the catalyst loaded asrings having an 8 mm (nominal) outside diameter into tubes having a 39mm inside diameter.

The catalyst for use in this invention additionally comprises a rheniumpromoter component deposited on the carrier in a quantity of greaterthan 1 mmole/kg, relative to the weight of the catalyst. Preferably, therhenium promoter may be present in a quantity of at least 0.5 mmole/kg,more preferably at least 1.5 mmole/kg, most preferably at least 2mmole/kg of the catalyst. Preferably, the rhenium promoter may bepresent in a quantity of at most 500 mmole/kg, more preferably at most50 mmole/kg, most preferably at most 10 mmole/kg, relative to the weightof the catalyst. Preferably, the rhenium promoter may be present in aquantity in the range of from 1.25 to 50 mmole/kg, more preferably from1.75 to 25 mmole/kg, most preferably from 2 to 10 mmole/kg, relative tothe weight of the catalyst. The form in which the rhenium promoter maybe deposited onto the carrier is not material to the invention. Forexample, the rhenium promoter may suitably be provided as an oxide or asan oxyanion, for example, as a rhenate or perrhenate, in salt or acidform.

The catalyst for use in this invention optionally comprises a firstco-promoter component. The first co-promoter may be selected fromsulfur, phosphorus, boron, and mixtures thereof. It is particularlypreferred that the first co-promoter comprises, as an element, sulfur.

The catalyst for use in this invention may additionally comprise asecond co-promoter component. The second co-promoter component may beselected from tungsten, molybdenum, chromium, and mixtures thereof. Itis particularly preferred that the second co-promoter componentcomprises, as an element, tungsten and/or molybdenum, in particulartungsten. The form in which first co-promoter and second co-promotercomponents may be deposited onto the carrier is not material to theinvention. For example, the first co-promoter and second co-promotercomponents may suitably be provided as an oxide or as an oxyanion, forexample, as a tungstate, molybdate, or sulfate, in salt or acid form.

The total quantity of the first co-promoter and the second co-promoterdeposited on the carrier is at most 10.0 mmole/kg, calculated as thetotal quantity of the elements (i.e., the total of sulfur, phosphorous,boron, tungsten, molybdenum and/or chromium) relative to the weight ofthe catalyst. Preferably, the total quantity of the first co-promoterand the second co-promoter may be at most 4.0 mmole/kg, more preferablyat most 3 mmole/kg of catalyst. Preferably, the total quantity of thefirst co-promoter and the second co-promoter may be at least 0.1mmole/kg, more preferably at least 0.5 mmole/kg, most preferably atleast 1 mmole/kg of the catalyst.

In an embodiment, the molar ratio of the first co-promoter to the secondco-promoter may be greater than 1. In this embodiment, the molar ratioof the first co-promoter to the second co-promoter may preferably be atleast 1.25, more preferably at least 1.5, most preferably at least 2, inparticular at least 2.5. The molar ratio of the first co-promoter to thesecond co-promoter may be at most 20, preferably at most 15, morepreferably at most 10.

In an embodiment, the molar ratio of the rhenium promoter to the secondco-promoter may be greater than 1. In this embodiment, the molar ratioof the rhenium promoter to the second co-promoter may preferably be atleast 1.25, more preferably at least 1.5. The molar ratio of the rheniumpromoter to the second co-promoter may be at most 20, preferably at most15, more preferably at most 10.

The catalyst may preferably also comprise a further element deposited onthe carrier. Eligible further elements may be selected from nitrogen,fluorine, alkali metals, alkaline earth metals, titanium, hafnium,zirconium, vanadium, thallium, thorium, tantalum, niobium, gallium,germanium, and mixtures thereof. Preferably, the alkali metals areselected from lithium, potassium, rubidium and cesium. Most preferably,the alkali metal is lithium, potassium and/or cesium. Preferably, thealkaline earth metals are selected from calcium, magnesium and barium.Preferably, the further element may be present in the catalyst in atotal quantity of from 0.01 to 500 mmole/kg, more preferably from 0.05to 100 mmole/kg, the total quantity of the element relative to theweight of the catalyst. The further element may be provided in any form.For example, salts or hydroxides of an alkali metal or an alkaline earthmetal are suitable. For example, lithium compounds may be lithiumhydroxide or lithium nitrate.

In an embodiment, the catalyst may preferably further comprise apotassium promoter deposited on the carrier. The additional potassiumpromoter is preferred especially when the carrier utilized in making thecatalyst contains low levels of leachable potassium. For example, theadditional potassium promoter is especially preferred when the carriercontains nitric acid leachable potassium in a quantity of less than 85ppmw, relative to the weight of the carrier, suitably at most 80 ppmw,more suitably at most 75 ppmw, most suitably at most 65 ppmw, samebasis. The additional potassium promoter is especially preferred whenthe carrier contains water leachable potassium in a quantity of lessthan 40 ppmw, relative to the weight of the carrier, suitably at most 35ppmw, more suitably at most 30 ppmw. In this embodiment, the potassiumpromoter may be deposited in a quantity of at least 0.5 mmole/kg,preferably at least 1 mmole/kg, more preferably at least 1.5 mmole/kg,most preferably at least 1.75 mmole/kg, calculated as the total quantityof the potassium deposited relative to the weight of the catalyst. Thepotassium promoter may be deposited in a quantity of at most 20mmole/kg, preferably at most 15 mmole/kg, more preferably at most 10mmole/kg, most preferably at most 5 mmole/kg, on the same basis. Thepotassium promoter may be deposited in a quantity in the range of from0.5 to 20 mmole/kg, preferably from 1 to 15 mmole/kg, more preferablyfrom 1.5 to 7.5 mmole/kg, most preferably from 1.75 to 5 mmole/kg, onthe same basis. A catalyst prepared in accordance with this embodimentcan exhibit an improvement in selectivity, activity, and/or stability ofthe catalyst especially when operated under conditions where thereaction feed contains low levels of carbon dioxide, describedhereinafter.

In an embodiment, the catalyst may preferably contain a quantity ofpotassium such that the amount of water extractable potassium of thecatalyst may be at least 1.25 mmole/kg, relative to the weight of thecatalyst, suitably at least 1.5 mmole/kg, more suitably at least 1.75mmole/kg, same basis. Suitably, the catalyst may contain waterextractable potassium in a quantity of at most 10 mmole/kg, moresuitably at most 7.5 mmole/kg, most suitably at most 5 mmole/kg, samebasis. Suitably, the catalyst may contain water extractable potassium ina quantity in the range of from 1.25 to 10 mmole/kg, more suitably from1.5 to 7.5 mmole/kg, most suitably from 1.75 to 5 mmole/kg, same basis.The source of water extractable potassium may originate from the carrierand/or the catalytic components. It is important to select a targetvalue for potassium for the entire catalyst composition (carrier plusadded catalyst components). For example if the target water extractablequantity of potassium is 10 mmole/g, relative to the weight of thecatalyst, such target potassium level is achieved by measuring thepotassium level of the carrier and adding sufficient additionalpotassium during the catalyst impregnation to achieve the targetpotassium level. A similar process for adding sodium could be applied inorder to achieve the proper target level.

The quantity of water extractable potassium in the catalyst is deemed tobe the quantity insofar as it can be extracted from the catalyst. Theextraction involves extracting a 2-gram sample of the catalyst threetimes by heating it in 25-gram portions of de-ionized water for 5minutes at 100° C. and determining in the combined extracts the amountof potassium by using a known method, for example atomic absorptionspectroscopy.

As used herein, unless otherwise specified, the quantity of alkali metalpresent in the catalyst and the quantity of water leachable componentspresent in the carrier are deemed to be the quantity insofar as it canbe extracted from the catalyst or carrier with de-ionized water at 100°C. The extraction method involves extracting a 10-gram sample of thecatalyst or carrier three times by heating it in 20 ml portions ofde-ionized water for 5 minutes at 100° C. and determining in thecombined extracts the relevant metals by using a known method, forexample atomic absorption spectroscopy.

As used herein, unless otherwise specified, the quantity of alkalineearth metal present in the catalyst and the quantity of acid leachablecomponents present in the carrier are deemed to be the quantity insofaras it can be extracted from the catalyst or carrier with 10% w nitricacid in de-ionized water at 100° C. The extraction method involvesextracting a 10-gram sample of the catalyst or carrier by boiling itwith a 100 ml portion of 10% w nitric acid for 30 minutes (1 atm., i.e.101.3 kPa) and determining in the combined extracts the relevant metalsby using a known method, for example atomic absorption spectroscopy.Reference is made to U.S. Pat. No. 5,801,259, which is incorporatedherein by reference.

Epoxidation Process

Although the present epoxidation process may be carried out in manyways, it is preferred to carry it out as a gas phase process, i.e. aprocess in which the feed is contacted in the gas phase with thecatalyst which is present as a solid material, typically in a packedbed. Generally the process is carried out as a continuous process.

The olefin for use in the present epoxidation process may be any olefin,such as an aromatic olefin, for example styrene, or a di-olefin, whetherconjugated or not, for example 1,9-decadiene or 1,3-butadiene.Typically, the olefin is a mono-olefin, for example 2-butene orisobutene. Preferably, the olefin is a mono-α-olefin, for example1-butene or propylene. The most preferred olefin is ethylene. Suitably,mixtures of olefins may be used.

The quantity of olefin present in the feed may be selected within a widerange. Typically, the quantity of olefin present in the feed will be atmost 80 mole-%, relative to the total feed. Preferably, it will be inthe range of from 0.5 to 70 mole-%, in particular from 1 to 60 mole-%,on the same basis. As used herein, the feed is considered to be thecomposition which is contacted with the catalyst.

The present epoxidation process may be air-based or oxygen-based, see“Kirk-Othmer Encyclopedia of Chemical Technology”, 3^(rd) edition,Volume 9, 1980, pp. 445-447. In the air-based process, air or airenriched with oxygen is employed as the source of the oxidizing agentwhile in the oxygen-based processes high-purity (at least 95 mole-%) orvery high purity (at least 99.5 mole-%) oxygen is employed as the sourceof the oxidizing agent. Reference may be made to U.S. Pat. No.6,040,467, incorporated by reference, for further description ofoxygen-based processes.

The quantity of oxygen present in the feed may be selected within a widerange. However, in practice, oxygen is generally applied in a quantitywhich avoids the flammable regime. Typically, the quantity of oxygenapplied will be within the range of from 1 to 15 mole-%, more typicallyfrom 2 to 12 mole-% of the total feed. In order to remain outside theflammable regime, the quantity of oxygen present in the feed may belowered as the quantity of the olefin is increased. The actual safeoperating ranges depend, along with the feed composition, also on thereaction conditions such as the reaction temperature and the pressure.

A reaction modifier may be present in the feed for increasing theselectively, suppressing the undesirable oxidation of olefin or olefinoxide to carbon dioxide and water, relative to the desired formation ofolefin oxide. Many organic compounds, especially organic halides andorganic nitrogen compounds, may be employed as the reaction modifiers.Nitrogen oxides, organic nitro compounds such as nitromethane,nitroethane, and nitropropane, hydrazine, hydroxylamine or ammonia maybe employed as well. It is frequently considered that under theoperating conditions of olefin epoxidation the nitrogen-containingreaction modifiers are precursors of nitrates or nitrites, i.e. they areso-called nitrate- or nitrite-forming compounds. Reference may be madeto EP-A-3642 and U.S. Pat. No. 4,822,900, which are incorporated hereinby reference, for further description of nitrogen-containing reactionmodifiers.

Organic halides are the preferred reaction modifiers, in particularorganic bromides, and more in particular organic chlorides. Preferredorganic halides are chlorohydrocarbons or bromohydrocarbons. Morepreferably they are selected from the group of methyl chloride, ethylchloride, ethylene dichloride, ethylene dibromide, vinyl chloride or amixture thereof. Most preferred reaction modifiers are ethyl chloride,vinyl chloride and ethylene dichloride. Additional disclosure regardingreaction modifiers can be found in, for example, U.S. Pat. No.7,193,094.

Suitable nitrogen oxides are of the general formula NO_(x) wherein x isin the range of from 1 to 2, and include for example NO, N₂O₃ and N₂O₄.Suitable organic nitrogen compounds are nitro compounds, nitrosocompounds, amines, nitrates and nitrites, for example nitromethane,1-nitropropane or 2-nitropropane. In preferred embodiments, nitrate- ornitrite-forming compounds, e.g. nitrogen oxides and/or organic nitrogencompounds, are used together with an organic halide, in particular anorganic chloride.

The reaction modifiers are generally effective when used in smallquantities in the feed, for example up to 0.1 mole-%, relative to thetotal feed, for example from 0.01×10⁻⁴ to 0.01 mole-%. In particularwhen the olefin is ethylene, it is preferred that the reaction modifieris present in the feed in a quantity of from 0.1×10⁻⁴ to 500×10⁻⁴mole-%, in particular from 0.2×10⁻⁴ to 200×10⁻⁴ mole-%, relative to thetotal feed.

In addition to the olefin, oxygen and the reaction modifier, the feedmay contain one or more optional components, such as carbon dioxide,inert gases and saturated hydrocarbons. Carbon dioxide is a by-productin the epoxidation process. However, carbon dioxide generally has anadverse effect on the catalyst activity. Typically, a quantity of carbondioxide in the feed in excess of 25 mole-%, preferably in excess of 10mole-%, relative to the total feed, is avoided. A quantity of carbondioxide of less than 6 mole-%, preferably less than 3 mole-%, inparticular in the range of from 0.3 to less than 1 mole-%, relative tothe total feed, may be employed. Under commercial operations, a quantityof carbon dioxide of at least 0.1 mole-%, or at least 0.2 mole-%,relative to the total feed, may be present in the feed. Inert gases, forexample nitrogen or argon, may be present in the feed in a quantity offrom 30 to 90 mole-%, typically from 40 to 80 mole-%. Suitable saturatedhydrocarbons are methane and ethane. If saturated hydrocarbons arepresent, they may be present in a quantity of up to 80 mole-%, relativeto the total feed, in particular up to 75 mole-%. Frequently, they arepresent in a quantity of at least 30 mole-%, more frequently at least 40mole-%. Saturated hydrocarbons may be added to the feed in order toincrease the oxygen flammability limit.

The epoxidation process may be carried out using reaction temperaturesselected from a wide range. Preferably the reaction temperature is inthe range of from 150 to 325° C., more preferably in the range of from180 to 300° C.

The epoxidation process is preferably carried out at a reactor inletpressure in the range of from 1000 to 3500 kPa. “GHSV” or Gas HourlySpace Velocity is the unit volume of gas at normal temperature andpressure (0° C., 1 atm, i.e. 101.3 kPa) passing over one unit volume ofpacked catalyst per hour. Preferably, when the epoxidation process is agas phase process involving a packed catalyst bed, the GHSV is in therange of from 1,500 to 10,000 h⁻¹. Preferably, the process is carriedout at a work rate in the range of from 0.5 to 10 kmole olefin oxideproduced per m³ of catalyst per hour, in particular 0.7 to 8 kmoleolefin oxide produced per m³ of catalyst per hour, for example 5 kmoleolefin oxide produced per m³ of catalyst per hour. As used herein, thework rate is the amount of the olefin oxide produced per unit volume ofcatalyst per hour and the selectivity is the molar quantity of theolefin oxide formed relative to the molar quantity of the olefinconverted. Suitably, the process is conducted under conditions where theolefin oxide partial pressure in the product mix is in the range of from5 to 200 kPa, for example 11 kPa, 27 kPa, 56 kPa, 77 kPa, 136 kPa, and160 kPa. The term “product mix” as used herein is understood to refer tothe product recovered from the outlet of an epoxidation reactor.

The olefin oxide produced may be recovered from the product mix by usingmethods known in the art, for example by absorbing the olefin oxide froma reactor outlet stream in water and optionally recovering the olefinoxide from the aqueous solution by distillation. At least a portion ofthe aqueous solution containing the olefin oxide may be applied in asubsequent process for converting the olefin oxide into a 1,2-diol, a1,2-diol ether, a 1,2-carbonate, or an alkanolamine.

Conversion of Olefin Oxide to Other Chemicals

The olefin oxide produced in the epoxidation process may be convertedinto a 1,2-diol, a 1,2-diol ether, a 1,2-carbonate, or an alkanolamine.As this invention leads to a more attractive process for the productionof the olefin oxide, it concurrently leads to a more attractive processwhich comprises producing the olefin oxide in accordance with theinvention and the subsequent use of the obtained olefin oxide in themanufacture of the 1,2-diol, 1,2-diol ether, 1,2-carbonate, and/oralkanolamine.

The conversion into the 1,2-diol or the 1,2-diol ether may comprise, forexample, reacting the olefin oxide with water, suitably using an acidicor a basic catalyst. For example, for making predominantly the 1,2-dioland less 1,2-diol ether, the olefin oxide may be reacted with a tenfoldmolar excess of water, in a liquid phase reaction in presence of an acidcatalyst, e.g. 0.5-1.0% w sulfuric acid, based on the total reactionmixture, at 50-70° C. at 1 bar absolute, or in a gas phase reaction at130-240° C. and 20-40 bar absolute, preferably in the absence of acatalyst. The presence of such a large quantity of water may favor theselective formation of 1,2-diol and may function as a sink for thereaction exotherm, helping control the reaction temperature. If theproportion of water is lowered, the proportion of 1,2-diol ethers in thereaction mixture is increased. The 1,2-diol ethers thus produced may bea di-ether, tri-ether, tetra-ether or a subsequent ether. Alternative1,2-diol ethers may be prepared by converting the olefin oxide with analcohol, in particular a primary alcohol, such as methanol or ethanol,by replacing at least a portion of the water by the alcohol.

The olefin oxide may be converted into the corresponding 1,2-carbonateby reacting the olefin oxide with carbon dioxide. If desired, a 1,2-diolmay be prepared by subsequently reacting the 1,2-carbonate with water oran alcohol to form the 1,2-diol. For applicable methods, reference ismade to U.S. Pat. No. 6,080,897, which is incorporated herein byreference.

The conversion into the alkanolamine may comprise, for example, reactingthe olefin oxide with ammonia. Anhydrous ammonia is typically used tofavor the production of monoalkanolamine. For methods applicable in theconversion of the olefin oxide into the alkanolamine, reference may bemade to, for example U.S. Pat. No. 4,845,296, which is incorporatedherein by reference.

The 1,2-diol and the 1,2-diol ether may be used in a large variety ofindustrial applications, for example in the fields of food, beverages,tobacco, cosmetics, thermoplastic polymers, curable resin systems,detergents, heat transfer systems, etc. The 1,2-carbonates may be usedas a diluent, in particular as a solvent Alkanolamines may be used, forexample, in the treating (“sweetening”) of natural gas.

Unless specified otherwise, the low-molecular weight organic compoundsmentioned herein, for example the olefins, 1,2-diols, 1,2-diol ethers,1,2-carbonates, alkanolamines, and reaction modifiers, have typically atmost 40 carbon atoms, more typically at most 20 carbon atoms, inparticular at most 10 carbon atoms, more in particular at most 6 carbonatoms. As defined herein, ranges for numbers of carbon atoms (i.e.carbon number) include the numbers specified for the limits of theranges.

Illustrative Embodiments Preparation of Carrier Samples

Processes for manufacturing carriers for use in epoxidation reactionsare described in numerous publications including U.S. Pat. No. 5,100,859and U.S. Pat. No. 6,831,037 which are incorporated herein by reference.See, for example, the disclosure in U.S. Pat. No. 5,100,859 which beginsat column 2, line 6 and continues to column 6, line 43.

Carrier A First Comparative Example

Carrier A was prepared according to the teachings in U.S. Pat. No.5,100,859 that pertain to Carrier L. The alumina powder was combinedwith zirconia, magnesium silicate, walnut shell flour, boric acid andextrusion aid to create a mixture which was then extruded to form hollowcylinders that were dried and fired. The physical and chemicalcharacteristics of the fired cylinders, which may be referred to ascarriers or supports, were determined using standard analyticaltechniques and the data is shown below in Table I. Carrier A's waterabsorption was 0.489 g/g, the ratio of water absorption to surface areawas 0.64 g/m² and the attrition was 16.8%.

Carrier B Carrier of this Invention

Carrier B was made by following the disclosure in U.S. Pat. No.5,100,859 as it pertains to the process for manufacturing Carrier L,shown in Table 5, except for the use of burn-out, which was excluded.The incorporation of a coarse pore former, such as ground shells, may beavoided or limited to less than 0.01 weight percent based on the weightof the alumina. Preferably, the use of a pore former is avoided. Thealpha alumina used in the carrier preparation had a purity greater than98.5 weight percent and the level of certain impurities were controlledto insure that the carrier had the desired chemical composition. Usingan alumina powder that had a low amount of sodium led to a carrierwherein the amount of soda in the carrier was no more than 0.04 weightpercent. The alumina powder was mixed with magnesium silicate, whichfunctioned as a permanent bond, and contributed to elevating the totalquantity of silica in the carrier to at least 0.06 weight percent basedon the weight of the alumina. Unlike carrier L in U.S. Pat. No.5,100,859, Carrier B did not include walnut shell flour which may bereferred to as a coarse pore former. The lack of a pore former inCarrier B's formulation is believed to be the reason why Carrier B had amonomodal pore size distribution compared to Carrier A which included apore former and had a bimodal pore size distribution. The physical andchemical characteristics of Carrier B are shown below in Table 1.Carrier B's water absorption was 0.271 g/g, the ratio of waterabsorption to surface area was 0.36 g/m² and the attrition was 6.9%. TheNa₂O content was 0.02 weight percent and the silica content was 0.24weight percent.

Carrier C Second Comparative Example

Carrier C was a commercially available carrier provided by Saint-GobainNorPro of Stow, Ohio USA. This carrier's commercial designation is SA5202. The physical and chemical characteristics of an SA 5502 carrierare disclosed in U.S. Pat. No. 5,187,140 at column 43. The numericaldesignation 5202 indicates that the shape of the carrier is a spherewhile 5502 indicates that the shape of the carrier is a hollow cylinder.The numbers “02” are indicative of the formula used to make the carrierand thereby indicates that the 5202 spheres and 5502 cylinders were madeusing the same formula. The physical and chemical characteristics ofCarrier C, which are recorded in Table 1, reveal a water absorption of0.247 g/g, a ratio of water absorption to surface area of 0.29 g/m², asoda content of 0.10 percent by weight of the carrier and a silicacontent of 0.03 percent by weight of the carrier.

TABLE 1 Carrier Properties A B C comparative invention comparativeChemical Analysis^(a) Na₂O 0.03 0.02 0.10 SiO₂ 0.26 0.24 0.03 Pore SizeDistribution Bi-modal Mono- Mono- modal modal Physical Properties WaterAbsorption (gram of 0.489 0.271 0.247 water/gram of carrier) SurfaceArea (m²/g) 0.76 0.76 0.84 Ratio of Water Absorption 0.64 0.36 0.29 toSurface Area (g/m²) Bulk Packing Density 698 992 1148 kg/m³ (lbs/ft³)(43.6) (61.9) (71.7) Attrition Loss, % 16.8 6.9 NA^(b) Average FlatPlate Crush 86.7 116 NA^(b) Strength, N (lbf) (19.5) (26.0) ^(a)% wt ofthe carrier ^(b)data not available for comparison because Carrier C wasshaped as a sphere and Carriers A and B were hollow cylinders.

Analysis of the data in Table 1 reveals the following distinctionsbetween carriers A, B and C. In contrast to Carrier C which has a silicacontent of 0.03 weight percent and a soda content of 0.10 weightpercent, Carrier B, which is a carrier of this invention, has a silicacontent of 0.24 weight percent and a soda content of 0.02 weightpercent. Clearly, the silica content of Carrier B is well above thesilica content of Carrier C and the soda content of Carrier B is wellbelow the soda content of Carrier C. In contrast to Carrier A which hasa bimodal pore size distribution, a water absorption of 0.489 g/g and aratio of water absorption to surface area of 0.64 g/m², Carrier B hasmonomodal pore size distribution, a water absorption of 0.271 g/g and aratio of water absorption to surface area of 0.36 g/m². Carrier B'sunique combination of: (1) low water absorption, defined herein as lessthan 0.35 g/g; (2) high surface area, defined herein as greater than0.70 m²/g; (3) a silica content between 0.06 and 0.40% by weight of thecarrier; and (4) low soda content, defined herein as less than 0.04weight percent, are believed to contribute to the performance ofcatalysts made from Carrier B as demonstrated by the selectivity data inTable 4 which is shown below in the Catalysts Examples portion of thisspecification.

Shown in FIGS. 1A and 1B are the Total Pore Volume curves and theIncremental Pore Volume curves, respectively, for carriers A and Cwherein line 20 represents carrier A and line 22 represents carrier C.Shown in FIGS. 2A and 2B are the Total Pore Volume curve and theIncremental Pore Volume curve, respectively, for carrier B which isidentified as line 26. Shown below in Table 2 are the percentages oftotal pore volume with a specified range of pore diameters. The dataclearly illustrates that carrier A has a bimodal pore size distributionwith only 42% of the total pore volume contributed by pores having a netrange of less than 3.8 microns. In contrast, carriers B and C have 95%and 94%, respectively, of their total pore volumes contributed by poreswithin a net range of 3.8 microns. Furthermore, the monomodal nature ofcarrier B pore size distribution is visually discernable in FIG. 2A andobjectively quantified in Table 2 wherein 83% of carrier B's total porevolume is contributed by pores with a net range of 1.4 microns. Incontrast, only 33% of carrier A's total pore volume is contributed bypores within a net range of 1.4 microns.

TABLE 2 Lower Upper Net Range Pore Pore of Pore Diameter DiameterCarrier Designation Diameters Limit Limit A¹ B¹ C¹ 3.8 0.2 4.0 42 95 942.6 0.4 3.0 39 92 87 1.4 0.6 2.0 33 83 76 ¹Percentage of total porevolume within specified range.

Preparation of Catalyst Examples Preparation of Stock Silver Solution

Standard silver solutions were used in all catalyst preparations. Atypical solution composition range of major components before dilutionis 25-35 wt % Ag⁺, 15-20 wt % ethylene diamine, 10-14 wt % C₂O₄ ⁻² and40-50 wt % H₂O. In each of the following catalyst preparation examples,dopants and diluents were added to this stock solution to give the finalimpregnating solution. The amount of diluent added to the stock solutionwas based upon the stock solution specific gravity, the carrier waterabsorption, and the target silver loading for the final catalyst.

Preparation of Catalysts

The three carriers A, B and C described above were used in preparingcatalysts according to Examples 1-7.

Example 1 (Comparative) Preparation of Catalyst Based on Carrier A

Catalyst 1 was prepared by the following procedure: To 192.2 grams ofstock silver solution of specific gravity 1.549 g/ml was added 0.1793 gof ammonium perrhenate in 2 g of 1:1 ethylenediamine/water; 0.0500 g ofammonium metatungstate dissolved in 2 g of 1:1 ammonia/water; 0.0855 gof ammonium sulfate dissolved in 2 g of water; 0.2664 g of lithiumhydroxide monohydrate dissolved in water, and 0.0676 g potassium nitratedissolved in 2 g water. Additional water was added to adjust thespecific gravity of the solution to 1.501 g/ml. 50 g of the resultingsolution was mixed with 0.1045 g of 50% w cesium hydroxide solution,producing the final impregnation solution. A vessel containing 30 gramsof Carrier A hollow cylinders was evacuated to 20 mm Hg for 1 minute andthe final impregnation solution was added to the carrier pellets whileunder vacuum, then the vacuum was released and the carrier allowed tocontact the liquid for 3 minutes. The impregnated carrier was thencentrifuged at 500 rpm for 2 minutes to remove excess liquid. The wetcarrier pellets were placed in a vibrating shaker and dried in airflowing at a rate of 16.2 Nl/h at 250° C. for 5.5 minutes producingCatalyst 1.

The final composition of Catalyst 1 comprised the following, calculatedon the basis of pore volume impregnation: 17.5% w silver; 2.0 micromoleRe/g; 0.6 micromole W/g; 2.0 micromole S/g; 21 micromole Li/g; 2.0micromole K/g, and 4.5 micromole Cs/g. These values are relative to theweight of the catalyst.

Example 2 (Inventive) Preparation of Catalyst Based on Carrier B

Catalyst 2 was prepared in two impregnation steps. Approximately 120grams of Carrier B was first impregnated with 204 grams of silversolution having a specific gravity of 1.478 g/cc according to theprocedure for Catalyst 1, except that no dopants were added to thesilver solution. The resulting dried catalyst precursor containedapproximately 9.8 wt % silver. The dried Catalyst 2 Precursor was thenimpregnated with a second solution which was made by mixing 191.0 gramsof silver stock solution of specific gravity 1.55 g/cc with a solutionof 0.3375 g of NH₄ReO₄ in 2 g of 1:1 EDA/H₂O, 0.0941 g of ammoniummetatungstate dissolved in 2 g of 1:1 ammonia/water, 0.1610 gLi₂SO₄.H₂O, 0.1272 g KNO₃, and 0.5015 g LiOH.H₂O dissolved in water.Additional water was added to adjust the specific gravity of thesolution to 1.478 g/cc. 50 grams of such doped solution was mixed with0.2109 g of 44.8 wt % CsOH solution. This final impregnation solutionwas used to prepare Catalyst 2. A flask containing 30 grams of theCatalyst 2 Precursor was evacuated to 20 mm Hg for 1 minute and thefinal impregnation solution was added while under vacuum, then thevacuum was released and the precursor allowed to contact the liquid for3 minutes. The impregnated precursor was then centrifuged at 500 rpm for2 minutes to remove excess liquid. The wet Catalyst 2 pellets wereplaced in a vibrating shaker and dried in air flowing at a rate of 460SCFH at 250° C. for 5.5 minutes. The final Catalyst 2 composition was17.5% Ag, 600 ppm Cs/g catalyst, 2.0 μmole Re/g catalyst, 0.60 μmole W/gcatalyst, 2.0 S/g catalyst, 2.0 micromole K/g catalyst, and 21 μmoleLi/g catalyst.

Example 3 (Inventive) Preparation of Catalyst Based on Carrier B

Catalyst 3 was prepared in three impregnation steps. Approximately 120grams of Carrier B was first impregnated with 204 grams of silversolution having a specific gravity of 1.549 g/cc according to theprocedure for catalyst 1, except that no dopants were added to thesilver solution. The impregnation/centrifuge/drying procedure wasperformed a total of two times, resulting in a dried catalyst precursorcontaining approximately 19.5 wt % silver. The dried Catalyst 3Precursor was then impregnated with a final solution which was made bymixing 191.9 grams of silver stock solution of specific gravity 1.549g/cc with a solution of 0.3962 g of NH₄ReO₄ in 2 g of 1:1 EDA/H₂O,0.1105 g of ammonium metatungstate dissolved in 2 g of 1:1ammonia/water, 0.1890 g Li₂SO₄.H₂O, 0.1493 g KNO₃, and 0.5888 g LiOH.H₂Odissolved in water. Additional water was added to adjust the specificgravity of the solution to 1.500 g/cc. To 50 grams of such dopedsolution was added 0.2160 g of 47.02 wt % CsOH solution. This finalimpregnation solution was used to prepare Catalyst 3. A flask containing30 grams of the Catalyst 3 Precursor was evacuated to 20 mm Hg for 1minute and the final impregnation solution was added while under vacuum,then the vacuum was released and the precursor allowed to contact theliquid for 3 minutes. The impregnated precursor was then centrifuged at500 rpm for 2 minutes to remove excess liquid. Catalyst 3 pellets wereplaced in a vibrating shaker and dried in air flowing at a rate of 460SCFH at 250° C. for 5.5 minutes. The final Catalyst 3 composition was25.7% Ag, 550 ppm Cs/g catalyst, 2.0 μmole Re/g catalyst, 0.60 μmole W/gcatalyst, 2.0 S/g catalyst, 2.0 micromole K/g catalyst, and 21 μmoleLi/g catalyst.

Example 4 (Comparative) Preparation of Catalyst Based on Carrier C

Catalyst 4 was prepared in two impregnation steps. Approximately 250grams of Carrier C was first impregnated with 370 grams of silversolution having a specific gravity of 1.478 g/cc according to theprocedure for Catalyst 1, except that no dopants were added to thesilver solution. The resulting dried catalyst precursor containedapproximately 9.0 wt % silver. The dried Catalyst 4 Precursor was thenimpregnated with a second solution which was made by mixing 370.5 gramsof silver stock solution of specific gravity 1.554 g/cc with a solutionof 0.5112 g of NH₄ReO₄ in 2 g of 1:1 EDA/H₂O, 0.1420 g of ammoniummetatungstate dissolved in 2 g of 1:1 ammonia/water, 0.2095 gLi₂SO₄.H₂O, 0.1927 g KNO₃, and 0.7561 g LiOH.H₂O dissolved in water.Additional water was added to adjust the specific gravity of thesolution to 1.478 g/cc. 50 grams of such doped solution was mixed with0.2438 g of 47.0 wt % CsOH solution. This final impregnation solutionwas used to prepare Catalyst 4. A flask containing 30 grams of theCatalyst 4 Precursor was evacuated to 20 mm Hg for 1 minute and thefinal impregnation solution was added while under vacuum, then thevacuum was released and the precursor allowed to contact the liquid for3 minutes. The impregnated precursor was then centrifuged at 500 rpm for2 minutes to remove excess liquid. The wet Catalyst 4 pellets wereplaced in a vibrating shaker and dried in air flowing at a rate of 460SCFH at 250° C. for 5.5 minutes. The final Catalyst 4 composition was16.2% Ag, 675 ppm Cs/g catalyst, 2.75 μmole Re/g catalyst, 0.822 μmoleW/g catalyst, 2.75 S/g catalyst, 2.75 micromole K/g catalyst, and 29μmole Li/g catalyst.

Example 5 (Comparative) Preparation of Catalyst Based on Carrier C

Catalyst 5 was prepared using the solution prepared according to theprocedure in Example 4, except that 0.2167 g of 47.0 wt % CsOH solutionwas used, resulting in a final cesium content of 600 ppmw.

Example 6 (Comparative) Preparation of Catalyst Based on Carrier C

Catalyst 6 was prepared using the solution prepared according to theprocedure in Example 4, except that 0.1626 g 47.0 wt % CsOH solution wasused, resulting in a final cesium content of 450 ppmw.

Example 7 (Comparative) Preparation of Catalyst Based on Carrier C

Catalyst 7 was prepared using the solution prepared according to theprocedure in Example 4, except that the dopant amounts were adjusted soas to give the final Catalyst 7 composition of 16.2% Ag, 600 ppm Cs/gcatalyst, 2.0 μmole Re/g catalyst, 0.60 μmole W/g catalyst, 2.0 S/gcatalyst, 2.0 micromole K/g catalyst, and 21 μmole Li/g catalyst.

TABLE 3 Compositions of Catalysts described in Examples 1-7. Dopantvalues reported in micromoles dopant per gram of catalyst. Ag loadingExample (wt %) Re W S Li K Cs 1 17.0 2.0 0.6 2.0 21 2.0 4.51 2 ^(a) 17.02.0 0.6 2.0 21 2.0 4.51 3 ^(a) 25.7 2.0 0.6 2.0 21 2.0 4.14 4 16.2 2.750.82 2.75 29 2.75 5.08 5 16.2 2.75 0.82 2.75 29 2.75 4.51 6 16.2 2.750.82 2.75 29 2.75 3.39 7 16.2 2.0 0.6 2.0 21 2.0 4.51 ^(a) According tothe invention

Catalyst Testing

The catalysts described above were used to produce ethylene oxide fromethylene and oxygen. To do this, 3 to 7 g of the crushed catalystsamples were loaded into separate stainless steel U-shaped tubes. Eachtube was immersed in a molten metal bath (heat medium) and the ends wereconnected to a gas flow system. The weight of catalyst used and theinlet gas flow rate were adjusted to give a gas hourly space velocity of3300 Nl/(l·h), as calculated for uncrushed catalyst. The inlet gaspressure was 1550 kPa (absolute).

Prior to startup, the catalysts were pre-treated for 3 hours with a gasmixture of 11.4 mole-% oxygen, 7 mole-% carbon dioxide and 81.6 mole-%nitrogen at 280° C. Then the reactor was cooled to 240° C., and testinggas mixture was introduced. The gas mixture passed through the catalystbed, in a “once-through” operation, during the entire test run includingthe start-up, consisted of 30.0 volume percent ethylene, 8.0 volumepercent oxygen, 5.0 volume percent carbon dioxide, 57 volume percentnitrogen and 0 to 6.0 parts per million by volume (ppmv) ethyl chloride.The temperature then adjusted so as to achieve a constant ethylene oxidecontent of 3.09 volume percent in the outlet gas stream. The quantity ofethyl chloride was varied to obtain maximum selectivity to ethyleneoxide. Initial performance data at this production level was measuredbetween 1 to 7 days of operation. The performance data is summarizedbelow in Table 4. Selectivity and temperature values corresponding toincreasing cumulative ethylene oxide production would also be measuredin order to obtain catalyst stability data.

TABLE 4 Properties and Performance of Catalysts from Examples 1-7. Ag AgLoading Ex- load- (kg Ag/m³ Cesi- Cesium am- Car- ing bulk um (ppmw/m²Sel. Temp. ple rier (wt %) catalyst) ^(a) (ppmw) carrier) (%) (° C.) 1 A17.0 140 600 789 87.5 272 2 ^(b) B 17.0 230 600 789 88.5 259 3 ^(b) B25.7 320 550 724 88.8 250 4 C 16.2 220 675 804 80.1 253 5 C 16.2 220 600714 80.7 247 6 C 16.2 220 450 536 84.7 250 7 C 16.2 220 600 714 79.8 257^(a) Based on loading of 8 mm (nominal outside diameter) rings in a 39mm (inside diameter) packed tube ^(b) According to the invention

It can be seen from the examples set forth that the inventive carriercan be used to make silver epoxidation catalysts with distinct advantageover prior art carriers. Comparing Examples 1 and 2, wherein thecarriers differ in water absorption, it is apparent that of catalystswith equivalent weight basis metal loading, the advantage in bothselectivity and activity is with the inventive carrier—that with anequivalent surface area but lower water absorption. A furtherimprovement in activity is found on the inventive carrier with increasedmetal loading (Examples 2 and 3). Examples 2 and 7 illustrate theremarkable difference between the inventive carrier and the prior artmaterial of similar physical properties. Examples 4, 5, and 6 are addedfor completeness in order to remove the question of dopant surfacedensity in the case of the slightly higher surface area of carrier C. Inthese examples, the dopants were increased by the same factor as thesurface area difference between Carrier B and Carrier C. Examples 2 and4 are the best comparison in this series, with very similar cesiumcontent. These data show that when we adjust the dopants to normalizethe surface area difference, there is still a clear advantage for thecatalyst made on inventive Carrier B. All the data taken togetherillustrate an advantage for carriers made with low water absorption, andof defined chemical composition.

In addition to the advantages found in initial performance of theinventive examples, certain embodiments of this invention could beuseful in providing catalysts with improved stability of selectivityand/or activity. Such improved longevity provides economic benefit forthe catalyst user.

The above description is considered that of particular embodiments only.Modifications of the invention will occur to those skilled in the artand to those who make or use the invention. Therefore, it is understoodthat the embodiments shown in the drawings and described above aremerely for illustrative purposes and are not intended to limit the scopeof the invention, which is defined by the following claims asinterpreted according to the principles of patent law, including theDoctrine of Equivalents.

What is claimed is:
 1. A carrier comprising at least 85 wt percent alphaalumina, at least 0.06 wt percent SiO₂ and no more than 0.04 wt percentNa₂O, said carrier comprising a water absorption no greater than 0.35gram of water/gram of carrier and a ratio of water absorption (gram ofwater/gram of carrier) to surface area (m² of carrier/gram of carrier)no greater than 0.50 gram of water/m² of carrier.
 2. The carrier ofclaim 1 wherein said SiO₂ does not exceed 0.40 wt percent.
 3. Thecarrier of claim 1 wherein said SiO₂ does not exceed 0.30 wt percent. 4.The carrier of claim 1 wherein said Na₂O does not exceed 0.03 wtpercent.
 5. The carrier of claim 1 wherein said carrier comprises atleast 0.15 wt percent SiO₂.
 6. The carrier of claim 1 wherein said ratiodoes not exceed 0.45 g/m².
 7. The carrier of claim 1 wherein said ratiodoes not exceed 0.40 g/m².
 8. The carrier of claim 1 wherein said waterabsorption does not exceed 0.30 g/g.
 9. The carrier of claim 8 whereinsaid surface area exceeds 0.70 m²/g.
 10. The carrier of claim 1 whereinsaid surface area exceeds 0.75 m²/g.
 11. The carrier of claim 1, whereinsaid carrier further comprises a total pore volume and at least 60% ofthe total pore volume is contributed by pores having diameters within arange no greater than 3.8 microns.
 12. The carrier of claim 1 wherein atleast 80% of the total pore volume is contributed by pores havingdiameters within a range no greater than 3.8 microns.
 13. The carrier ofclaim 1, wherein at least 90% of the total pore volume is contributed bypores having diameters within a range no greater than 3.8 microns. 14.The carrier of claim 1 wherein said carrier has a total pore volume andno more than 20% of said total pore volume is contributed by poresgreater than 10 microns.
 15. The carrier of claim 1 wherein no more than5 of said total pore volume is contributed by pores greater than 10microns.
 16. The carrier of claim 1 wherein no more than 10% of saidtotal pore volume is contributed by pores greater than 10 microns. 17.The carrier of claim 1 wherein said carrier comprises a bond material.18. The carrier of claim 1 wherein said carrier comprises non-plateletmorphology.